1. Field of the Invention
The present invention relates to a method for automatic temperature compensation of gas density gauges. Such gauges include cold and hot cathode type ionization gauges as well as heat loss gauges such as Pirani and thermocouple gauges. Such automatic temperature correction in the aforesaid gauges permits them to be more accurately used in the measurement of pressure. The concepts of the present invention can be applied to ionization gauges, heat loss gauges, and other types of gauges. The ionization type of gauge is discussed in greater detail hereunder.
Pressure measurement can result from the collision of gas molecules with a sensing surface. The pressure effect is a product of the number of molecules striking a unit surface per unit time and their average energy. The same pressure reading at a higher gas temperature where there is greater average energy per molecule, must thus have proportionately fewer gas molecules contacting the sensor per unit time.
A gas density gauge, however, measures only the relative number of molecules present--without concern for their average molecular energy. Thus, the density is an incomplete measure of the pressure, and those "pressure" gauges which measure only the density will be in error from this effect.
The universal gas law is shown in Eq. 1 where P is pressure, V is volume, n is moles of gas, EQU PV=nRT (Eq. 1)
R is a constant, and T is the absolute temperature. Gas density is in units of moles of gas/volume. Solving for density gives Equation 2. EQU density=n/V=(1/R)(P/T)=P/RT (Eq. 2)
The density is, therefore, proportional to the measured pressure divided by the measured temperature. Thus, the local density is inverse to the local absolute gas temperature. A density gauge can thus indicate a wide range of values at a fixed pressure, depending upon the absolute temperature of the gas in the sensitive part of the gauge at the time of making the measurement.
2. Description of the Prior Art
Devices are well-known in the prior art for use as temperature compensating pressure transducers which transduce pressure directly. Such pressure transducers transduce pressure directly into another variable such as displacement change or frequency change. This transduced variable may then be transduced into a second variable such as capacitance change, change in strain, and possibly even into several other variables before being transduced to an electrical voltage which is processed and displayed as the measured pressure. Each of these variables may be influenced by transducer temperatures and cause the measured pressure to be inaccurate by a fraction of a percent to several percent of the reading if the transducer temperature changes sufficiently. Therefore, for accurate pressure measurement, compensation must be provided for temperature induced changes in the physical dimensions of the transducer or changes in first, second or higher order variables. See for example, U.S. Pat. No. 4,607,530 to Chow wherein frequency changes are compensated.
The recent patent literature contains references to such aforementioned temperature compensated pressure gauges. The foregoing patents, which are discussed further hereunder, all seem to correct for the errors of the transducer only when it is operated at specified temperatures. Although the transducers measure pressure, they give incorrect indications at some temperatures.
The pressure is an independent variable--it should be possible to correctly measure pressure independently of the transducer temperature. Note that the situation under discussion is somewhat different from that of a gas trapped, as by a valve, in the transducer. The pressure of such a gas in a fixed volume does indeed respond to the absolute temperature of the gas, but that is not the case of concern here, though in Smalarz et al., U.S. Pat. No. 3,905,237, that principle is used to create compensating mechanisms for the range and zero of a pressure gauge, but not to make the fundamental measurement itself.
The U.S. Pat. No. 4,468,968 to Kee teaches that the temperature is that of the transducer assembly and also teaches of adjusting the voltage supply so that the electrical value of the output equals the assigned electrical value corresponding to the combination of parameter values. The patent to Kee does not consider gas density, per se, but rather refers broadly to this concept along with all other types of measurement.
U.S. Pat. No. 4,464,725 to Briefer works only with force systems, so the temperature correction is for the non-linearity of the transducer with temperature changes of the transducer.
Gross, in U.S. Pat. No. 4,399,515, also works with force systems, including those with multiple transducers. Thus, such corrections are also usable for transducer corrections with changing transducer temperature.
In the U.S. Pat. No. 4,392,382, to Meyers, there are taught direct pressure sensing capacitance manometers. The temperature related corrections are for transducer temperature correction.
Kurtz, in U.S. Pat. No. 4,192,005 teaches use of strain gauges and pertains to measurement of pressures. The temperature correction removes the sensitivity and zero shifts caused by changes in transducer temperature.
Pearson in U.S. Pat. No. 4,000,643 also uses strain gauges, and also pertains to pressure, and corrections taught therein are for transducer temperature problems.
Heise in U.S. Pat. No. 3,004,434 works with Bourdon tube gauges which sense pressure directly. This patent appears to be limited very specifically to such technology.
Cucci in U.S. Pat. No. 4,598,381 covers a differential pressure sensor (or set of sensors) and applies corrections to these only.
Yamada et al., U.S. Pat. No. 4,556,807, teaches a semiconductor diaphragm which responds to pressure. Located on the diaphragm is also a temperature sensor to correct the transducer response for changes in the diaphragm temperature.
Chow in U.S. Pat. No. 4,607,530, teaches a fast acting thermal sensor to determine the temperature at one point on a quartz crystal resonator gauge. The gauge senses pressure, and the thermal data is used in conjunction with a computer model to establish its effect on the pressure reading.
Scott, in U.S. Pat. No. 4,084,248, also teaches a general method for correcting errors from any transducer due to an independent error source, such as temperature. However, the technology employed is a specific type of look-up table that is outside the field of the present invention.
Juanarena, in U.S. Pat. No. 4,644,482, is a vibrating cylinder pressure gauge with a means for detecting the temperature of the transducer.
In the foregoing references a number of pressure gauges are taught, i.e., gauges that respond to force on a given area. These gauges have no fundamental temperature errors caused by the pressure changing with temperature. Their mechanisms are responsive to temperature changes by changing of the zero point of the gauge itself and by any changes in the response of the transducer used therewith. These changes can result from temperature changes occurring in various parts of the device, and have no direct relationship to the temperature of the gas.
The situation regarding density sensitive gauge elements, however, is just the reverse. The indication of such a gauge does not respond to the temperature of the gauge assembly by any measurable extent, but only to the temperature of the gas itself. The various electrodes in these density gauges can be at widely different temperatures--from white hot to near room temperature. Most often these gauges are of a very open format such that gas is not retained by any of the elements in any sense. If the temperature of one element of such a gauge is chosen as being representative of the gas temperature, this can generate an inconsistency if that element is not heated almost solely by the gas, or unless it restricts gas motion such that it totally determines the temperature of the contained gas. Such situations will usually occur only if intended by careful design.
Direct pressure transducers are rarely used for sensing pressures below pressures of about 1.times.10.sup.-3 Torr, and are rarely used for directly transducing pressure to another physical variable such as displacement. The force per unit area exerted by the rarified gas at low pressures is too small to be simply and conveniently transduced into an easily measurable displacement (of an indicator needle, for example) or other similar variables, using presently available technology.
To measure pressures below about 1.times.10.sup.-3 Torr conveniently and simply, gauges such as ionization gauges and discharge gauges are commonly used. The output of ionization and discharge gauges as well as that of other types, such as Pirani gauges, depends not on the gas pressure but rather on the gas density in the transducer. The output of these types of gauges depends on the number of atoms or molecules of gas present in the transducer and thus on the gas density. The very name "ionization gauge" implies action on individual molecules of gas. Such gauges are herein termed "gas density dependent pressure transducers" to distinguish them from direct pressure transducers such as strain gauges.
It is well-known that gas density varies inversely with the absolute temperature of the gas in different parts of the system. When the gas temperature rises in one portion of the system relative to another, the gas density in the hotter portion decreases relative to that in the cooler portion. To provide an accurate pressure measurement using a gas density dependent transducer, the gas density in the transducers must be corrected for the effects of gas temperature. This is to be contrasted with temperature compensation for direct pressure transducers wherein the correction is applied because of temperature induced changes in the properties of the transducer itself--not because of changes in the medium being measured.
Existing gas density dependent transducers are commonly calibrated against a pressure standard and serve to measure gas pressure reasonably well only so long as the gas temperature in the transducer remains the same as was the gas temperature in the transducer during calibration. If the gas temperature changes, significant errors will exist in the pressure measurement. For example, a gas temperature change of 30.degree. C. from calibration to actual use will produce approximately an 8% error in the pressure measurement in a typical ionization gauge and a 10% error in a typical Pirani gauge. At a typical bakeout temperature of 450.degree. C., the pressure measurement error using an ionization gauge is approximately 50% due to gas density change. Proportional errors exist for temperature changes intermediate to those cited above. Such errors cannot be ignored for many applications.
It has been well-known for almost two centuries that gas density varies inversely with absolute temperature. It has been known since the invention of the Pirani gauge circa 1906 that such gauges transduce gas density and not pressure. Thus, it has been known for many years that such errors exist in gas density dependent transducers. Heretofore, there has not previously been made a serious effort to correct for such errors. Indeed, it appears that such errors have sometimes been treated as inconsequential when they are indeed not inconsequential.
In National Bureau of Standards Note 298, dated Feb. 3, 1967, page 27, in analyzing errors in ionization vacuum gauges, the author states " . . . It is generally assumed, at least to a first approximation, that the rate of ionization in the gauge is proportional to the gas density. Safely, minor changes in envelope temperatures will not appreciably affect the rate of ionization, and therefore not affect the gauge indication. Two assumptions will be made: first that the gas and envelope temperatures are the same, and second, that equilibrium conditions exist in the gauge and vacuum system. If the temperature of the envelope is changing, adsorption and desorption and degassing in the gauge are far more significant on the indication of the ionization gauge than the direct effect of changes in gas temperatures."
The author of the above quote further states that if "the envelope temperature differs from that at which the gauge was calibrated, the pressure" P will equal P.sub.i (T/T.sub.o), where P.sub.i is the indicated pressure, T.sub.o is the envelope temperature at which the gauge was calibrated, and T is the envelope temperature. The author further states "Ordinarily this correction is not applied, first because the additional accuracy obtained is usually insignificant, and second, because envelope and gas temperatures are not necessarily related, particularly in a hot cathode gauge. It is impractical to measure the gas temperatures."
Ionization gauges are commonly calibrated in air conditioned laboratories such as exist at the National Bureau of Standards. They are often then placed in use deep within a complex vacuum system next to large horsepower mechanical pumps or next to high wattage diffusion pumps where the prevailing temperature is often at least 60.degree. to 70.degree. C. Thus, the temperature change is often about 40.degree. C. from calibration to actual use. This change will cause an error in a typical gauge of about 10% and in a typical Pirani gauge of about 14%. Such errors are significant for many applications.
In addition to changes in gas temperature induced by changes in ambient temperatures, there are larger changes induced by changes in cathode heater power. When the cathode (the electron emitter) is clean and new, the power required to produce a constant electron emission is minimal. When the cathode is contaminated or near the end of its life, the required power may increase several times. Thus, the gas temperature may change significantly because of changes in the power required to maintain a constant electron emission. When such temperature changes are added to typical ambient temperature changes, pressure measurement errors of 20 to 30% can easily exist in gas density dependent gauges.
A possible reason such errors have been ignored is that for many vacuum process applications it is not strictly necessary to know the exact value of the pressure. It is only necessary to be assured that the gas pressure, whatever it may be, is the same in the process from run to run. Thus, it may have been erroneously assumed that as long as the gauge indication was the same run to run that the gas pressure remained the same run to run. This assumption is incorrect as discussed hereunder.
Clearly, there is a need for a convenient and reliable method for the temperature compensating of gas density dependent pressure transducers.
It is an object of the present invention to provide a method for compensating density dependent pressure gauges for temperature changes.
A further object of the present invention is to provide a method for automatically and accurately providing a pressure measurement that is temperature compensated.